To estimate the reproductive schedule of the Okinawa spiny rat on the basis of its body size distribution, we developed a nest box with an auto-trigger camera. Three camera-installed nest boxes were used in the field in the northern part of Okinawa Island beginning in September 2015. The distance between eyes, an index of body size, was measured in 850 images of spiny rats. The reproductive schedule was estimated based on the emergence period of subadults. With the emergence of subadults, the distribution of body size index becomes a mixture of two normal distributions consisting of adults and subadults, so a Gaussian mixture model was employed to identify each distribution. The Gaussian mixture model revealed that the distribution of the estimated distance between eyes could be divided into two clusters, with a mixing ratio of 0.11 for Group 1 (subadult) and 0.89 for Group 2 (adult). Most individuals belonging to Group 1 appeared during the winter months of December to March, indicating that subadults were weaned primarily during this season. Consequently, reproduction of the Okinawa spiny rat is thought to be limited in autumn and winter.